Known in the art is an internal combustion engine arranging in an engine exhaust passage an NOX storing catalyst storing NOX contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NOX when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich. In this internal combustion engine, the NOX generated when burning fuel under a lean air-fuel ratio is stored in the NOX storing catalyst. On the other hand, if the NOX storing capability of the NOX storing catalyst approaches saturation, the air-fuel ratio of the exhaust gas is temporarily made rich and thereby NOX is released from the NOX storing catalyst and reduced.
However, fuel and lubrication oil contain sulfur, therefore the exhaust gas contains SOX. This SOX is stored together with the NOX in the NOX storing catalyst. However, this SOX is not released from the NOX storing catalyst by just making the air-fuel ratio of the exhaust gas rich, therefore the amount of SOX stored in the NOX storing catalyst gradually increases. As a result, the amount of NOX which can be stored ends up gradually decreasing.
Therefore, to prevent SOX from being sent to the NOX storing catalyst, an internal combustion engine provided with an SOX trap agent in the engine exhaust passage upstream of the NOX storing catalyst is known (see Japanese Patent Publication (A) No. 2004-92524). In this internal combustion engine, the SOX contained in the exhaust gas is trapped by the SOX trap agent, therefore the flow of SOX into the NOX storing catalyst is inhibited. As a result, it is possible to prevent the storing capability of NOX from falling due to the storage of SOX.
However, in this internal combustion engine, before the SOX trap capability of the SOX trap agent becomes saturated, SOX is released from the SOX trap agent. In this case, to ensure that the SOX is released well from the SOX trap agent, it is necessary to make the air-fuel ratio of the exhaust gas rich when the temperature of the SOX trap agent is the SOX release temperature. Therefore, in this internal combustion engine, to ensure that SOX be released from the SOX trap agent, the air-fuel ratio of the exhaust gas is made rich when the temperature of the SOX trap agent is the SOX release temperature.
Further, this internal combustion engine is provided with a bypass exhaust passage bypassing the NOX storing catalyst for preventing the SOX released from the SOX trap agent from flowing into the NOX storing catalyst. When SOX trap agent has released SOX, the exhaust gas flowing out from the SOX trap agent is guided into the bypass exhaust passage.
On the other hand, in this internal combustion engine, when the SOX trap amount of the SOX trap agent becomes a fixed amount or more, when the air-fuel ratio of the exhaust gas is made rich to release NOX from the NOX storing catalyst, even if the temperature of the SOX trap agent is made the SOX release temperature or less, SOX is released from the SOX trap agent, therefore the SOX ends up being stored in the NOX storing catalyst. Therefore, in this internal combustion engine, when the SOX trap amount of the SOX trap agent becomes a fixed amount or more, the air-fuel ratio of the exhaust gas is prohibited from being made rich.
However, in this case, if using an SOX trap agent not releasing SOX when the air-fuel ratio of the exhaust gas is made rich to release NOX from the NOX storing catalyst, SOX will no longer be stored in the NOX storing catalyst. Further, at any time when the NOX storing catalyst should release NOX, the air-fuel ratio of the exhaust gas can be made rich. However, so long as the SOX trap agent is given the function of releasing SOX like with this internal combustion engine, it is difficult to prevent SOX from being released when the air-fuel ratio of the exhaust gas becomes rich.
As opposed to this, if not giving the SOX trap catalyst the function of releasing SOX and only giving it the function of storing SOX, even if making the air-fuel ratio of the exhaust gas rich so as to make the NOX storing catalyst release NOX, usually SOX is not released from the SOX trap catalyst, therefore SOX is no longer stored in the NOX storing catalyst. However, the problem arises that even when using such an SOX trap catalyst, if the temperature of the SOX trap catalyst becomes the SOX release lower limit temperature or more, SOX will be released from the SOX trap catalyst when the air-fuel ratio of the exhaust gas is made rich so as to release NOX from the NOX storing catalyst.